Photo/Illutration The since-lifted tsunami advisory for the islands of Izu and Ogasawara on Sept. 24 (Captured from the Japan Meteorological Agency website)

Although the tsunami advisory for the Izu and Ogasawara islands was lifted at 11 a.m. on Sept. 24, one expert says residents and the main island of Honshu should remain on alert. 

The Japan Meteorological Agency's 8:20 a.m. advisory followed an earthquake at around 8:14 a.m. near Tokyo's uninhabited Torishima island that morning.

According to the JMA, the focus of the quake is estimated to be about 19 kilometers deep and the magnitude is estimated to be 5.8 on the Japanese seismic intensity scale of 7. The preliminary figure for the focus of the quake was 10 km deep. 

Tsunami of 50 centimeters and 20 cm were observed on Hachijojima island and Kozushima island, respectively.

Fumihiko Imamura, a tsunami engineering professor at Tohoku University, believes it is highly likely that it was triggered by undersea volcanic activity in the surrounding waters.

According to the JMA, earthquakes associated with volcanic activity have been continuously observed in the waters near Torishima island, including a magnitude-6.5 earthquake on Oct. 5, 2023, and a magnitude-5.7 earthquake in May 2018.

Imamura asserts the regularity of this activity lends to the high possibility it caused the most recent quake.

“Although the (latest) earthquake was relatively small at magnitude 5.9, its shallow depth of 10 km led to the tsunami,” Imamura said.

The professor explained that if the area of an earthquake that generates a tsunami is small and localized, the period of time a tsunami occurs tends to be relatively short, ranging from five to 10 minutes.

Meanwhile, tsunami caused by volcanic activity are reflected and transmitted from a narrow area and can last for a significant amount of time across a wide area.

Such volcanic activity is expected to continue, Imamura said, urging caution for residents in the Kanto to Tokai regions and on the Pacific side of Shikoku.

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